Heterojunction Carbon Nanotubes And Fullerene Adsorption Properties And The Growth Mechanism Of The Computer Simulation Study

Nano science and technology (NST) are at the forefront of technological revolutions in the 21st century, among which carbon nanomaterials have been the hot topics of many researchers, with special attention to the fullerenes and carbon nanaotubes (CNTs). The investigations on the adsorption properties especially associated with hydrogen storage properties and the growth mechanism for fullerenes and carbon nanaotubes are two most active fields, in which the theoretical study holds its distinguishable position as this in turn will contribute to experimental processes even practical applications.A large body of open literatures has addressed the issues related to the absorption properties of straight CNTs and the results showed that the absorption properties are highly dependent of their structure and morphology. Unfortunately, to our best knowledge, little coverage has been given to hetero-junctioned CNTs such as Y-junction CNTs that are available in fairly large amount depending on the methods and precursors used. To assess the possibility of non-straight or hetero-junctioned carbon nanotubes as adsorbents for several typical molecules, such as H₂, O₂, NH3, H2O, CH4, theoretical studies were performed on Y-shaped, L-shaped and straight CNTs via molecular mechanics (MM) and molecular dynamics (MD) simulations. For each shape, three different ideal single-walled structures are of particular interest. The factors that will influence the absorption properties of CNTs have been taken into account, including the shape, diameter and helicity of CNTs, the defects as well as the adsorption temperature. The mechanism involved in adsorption process is another focus. The main results are summarized as below:(1) By natural adsorption, the hydrogen amount in each open-ended CNT is quite limited, and seems unlikely to meet the standard set by department of energy (DOE) of America even at 80 K; (2) Hydrogen storage capacity in close-ended hetero-junctioned CNTs is higher than in straight CNTs under the same conditions, and Y-CNTs are also in preference to L-CNTs. For each type of CNT, larger diameter, lower temperature and smaller defect are beneficial for hydrogen storage, and the armchair-branched CNTs are more favorable to hydrogen storage than zigzag-branched CNTs; (3) Of all the gas molecules involved, NH3 and H₂O show higher tendency to be absorbed on hetero-junctioned CNTs, which can be interpreted in terms of more charge transfer; (4) For both adsorption and non-absorption state, the frontier orbitals of hetero-junctioned CNTs arise mainly from the carbon atoms on heptagons and their neighbors. This enables us to suggest that the carbon atoms on heptagons and their neighbors may function as active sites for further chemical reaction, which is confirmed by the calculations of chemical addition of hydrogen atoms; (5) The energy gaps between two frontier orbitals for each hetero-junctioned CNTs diminish considerably after the molecule was adsorbed, and this phenomenon is more pronounced for armchair hetero-junctioned CNTs.The possibility of hydrogen storage in several fullerenes and their oxides (C₆0, C₆0O, C₆0O₂, C70) was also a concern of this work. By ab initio method, the effects of the cage size,...